1. Field of Invention
This invention relates to the field of lumber mill sawing. More specifically, the invention comprises a process for scanning a log while it moves through the infeed section of a sawing operation, manipulating the data thereby obtained, calculating the optimum pattern of boards to be obtained from the particular log, and using that information to position the logs, chipping heads and saws to cut the log into said optimum pattern.
2. Description of Prior Art
Converting trees in a forest into finished wood products is a lengthy process. The first step is cutting down the tree. Next, the limbs are removed. This leaves the long and slender trunk, commonly called a xe2x80x9cstem.xe2x80x9d The stem is then transversely cut into selected lengths, in an operation called xe2x80x9cstem bucking.xe2x80x9d A long stem may be cut into as many as four separate sections, which are then called xe2x80x9clogs.xe2x80x9d
The larger diameter logs are often used to make framing lumber (such as 2xc3x974""s, 2xc3x976""s, etc.). The key to optimizing the use of a given log is determining what type and length of boards may be cut from the log while producing the minimum amount of waste. In recent years, computers have been used to calculate the optimum solution. In order to employ a computer, however, one must know the external dimensions of the log in question. Thus, the surface characteristics of the log must be accurately measured.
Optical scanning technology of varying sophistication has long been employed to determine the characteristics of a given log. The earliest applications of this technology appear to be in the field of veneer lathes. A veneer lathe grips a log securely by its sawn ends and rotates it against a long knife, which peels the log into a layer of thin veneer. This is a relatively slow process, and it was therefore well-suited to the slow scanning and computing speeds of the 1970""s. A good example of this technology is found in U.S. Pat. No. 3,852,579 to Sohn et.al. (1974). The Sohn device maps the log surface as it is turned slowly on the spindle of the veneer lathe. Similar devices, particularly adapted to the veneer lathe, are illustrated in U.S. Pat. No. 4,246,940 to Edwards et.al. (1981), U.S. Pat. No. 4,383,560 to McGee (1983), U.S. Pat. No. 4,397,343 to Fields (1983), U.S. Pat. No. 4,884,605 to Ely (1989), and U.S. Pat. No. 5,518,052 to Westberg et.al. (1996). All these devices make use of the existing veneer lathe spindles to rotate the log.
The same approach has been applied to non-veneer systems; i.e., scanning to optimize sawing for lumber production. Illustrating this adaptation are U.S. Pat. No. 4,197,888 to McGee et.al. (1980), U.S. Pat. No. 4,867,213 to Bolton et.al. (1989), and U.S. Pat. No. 5,257,101 to Lee (1993). All these devices scan the surface of the log by rotating it between two spindles. While this technique is acceptable for a veneer lathe, which must be configured to grip and rotate the log in order to cut the veneer, it is impractical for normal lumber production. In normal lumber production, the logs are moved rapidly in a direction parallel to their long axis. The line speeds are typically around 300 to 400 feet per minute. A system which requires the log to remain stationary while it is rotated and scanned is not practical where line speeds are this high.
Because the conveying line in sawing operations is nearly always in motion, scanning methods that could work without stopping the log were developed. The earliest of these were simple xe2x80x9cprofilexe2x80x9d scanners, which were only capable of determining the silhouette of the log. Illustrating this stage in the development is U.S. Pat. No. 4,316,491 to Kearnes et.al. (1982). The Kearnes device scans the profile (or silhouette) of the log as it moves past. It is capable of determining the profile in one or two planes, and then using that crude data to shift the log in order to optimize lumber production as the log is fed into the saws. As the technology evolved, interpolation was applied to the silhouette data to produce an approximate (albeit crude) surface model of the entire log. This technique is illustrate by U.S. Pat. No. 4,879,659 to Bowlin et.al. (1989). The Bowlin device also uses a laser-based camera scanner to detect surface imperfections once the initial chipping operations are complete. The use of this simple surface scanner marks another step in the evolutionary progress of log scanning. However, the ""659 device only maps a cross section of the log by measuring four tangent points spaced 90 degrees apart (determined from silhouettes taken in two planes 90 degrees apart). The computer then generates an elliptical cross section matching these four points. Unfortunately, many logs have a much more complex cross-section. Thus, while the Bowlin device represents an advancement, it is still far from achieving complete optimization of the lumber process.
A similar scanning process is disclosed in U.S. Pat. No. 4,947,909 to Stroud (1990). The Stroud device uses profile scanning in two planes to determine a curved cutting path through the log in order to optimize lumber recovery. It also creates only a crude representation of the surface by measuring the log""s silhouette. Thus, like Bowlin, it represents a limited advance.
Yet another profiling type scanning system is shown in U.S. Pat. No. 5,414,268 to McGee (1995). The McGee device uses a complex array of lasers and sensors to rapidly determine the log""silhouette in two orthogonal planes. However, the log must be moved slowly into and out of the scanning field in a direction perpendicular to its long axis. As explained previously, such a system is impractical for most lumber sawing operations, where the logs are continuously moved in a direction parallel to their long axis.
U.S. Pat. No. 5,429,161 to Allard (1995) discloses a more sophisticated form of scanning. The Allard device scans the log after two planar faces have been cut on opposite sides (the resulting processed log is often called a xe2x80x9ccantxe2x80x9d). The scanners can detect the transition point between the rough natural surface of the log and the smooth planar face. This information is fed into a computer and used to optimize subsequent resawing operations on the cant. It is significant to note, however, that the prior chipping or sawing of the two planar surfaces in the creation of the cant in itself represents a loss of optimality in the entire process. Considerable wood may have been wasted in creating the planar surfaces before the scanning station is even reached. Thus, a system which can determine surface characteristics of the entire log prior to any wood removal would be much better than the device disclosed in Allard.
A real-time scanner adapted to process logs traveling in a direction parallel to their long axis is disclosed in U.S. Pat. No. 5,765,617 to Mierau et.al. (1998). The Mierau device uses two scanners to control the rotation and skew of a log being fed into chippers and saws for cutting dimensioned lumber. While this device is capable of scanning a log without interrupting its linear travel down the line, it does require an infeed conveyor having a length of at least twice the longest log length that will be fed into the system. This increase in the required conveyor length can be a significant disadvantage, especially where a scanning system is being added to an existing line. Extending the infeed conveyor to twice the length of the longest log may require reconfiguring the entire line to obtain the additional length. Given that most of the processing equipment is set permanently in place and weighs several hundred tons, this fact is a significant impracticality.
The known methods for scanning the surface of a log and using the data to optimize chipping and sawing operations are therefore limited in that they:
1. Require the log""s motion along the line to be stopped while the log is rotated and scanned;
2. Provide only profile, or silhouette data, rather than true surface model;
3. Provide only a crude surface model by fitting circles or ellipses through four known tangent points along the log""s surface;
4. Only scan the log after two planar surfaces have been chipped away; and
5. Require a significant extension in the length of the infeed conveyor.
Accordingly, several objects and advantages of the present invention are:
1. To scan the log without the need for interruption in the log""s movement along the line;
2. To provide a full model of the log""s surface, rather than just silhouette data;
3. To model the log""s cross sections using enough surface points to accurately reflect the shape of the log, rather than using approximating circles or ellipse;
4. To scan and map the surface of the log before any chipping or sawing is performed;
5. To scan and map the log""s surface without requiring a substantially lengthened infeed conveyor;
6. To use the developed surface model of the log to compute the optimum solution of dimensioned lumber to be cut from the log; and
7. To provide a scanning and mapping system that is fast enough to allow the scanning of the log, generation of the surface model, computation of the optimum lumber solution, and movement of the chippers and saws, while the log is traveling at line speed.